With advances in ultra-low power IC (integrated circuits) design and MEMS (Microelectromechanical Systems) technologies, the size of electronic circuits and devices and the energy needed to drive them have been being reduced significantly. Harvesting energy from the environment to power microelectronics devices (e.g., mobile and wireless electronics) and MEMS devices has become a practical and alternative solution for replacing batteries. As energy harvesters produce power in the μW to mW range, outputting electric energy as much as possible is the primary goal of energy harvester development.
Energy harvesting or scavenging refers to the conversion of ambient energy such as thermal, radiant and kinetic energy into electrical energy. Vibration-to-electric energy conversion is one type of energy harvesting where ambient vibration energy is extracted and converted by a vibration energy harvester into electrical energy. There are three mechanisms for converting vibration energy, i.e., electromagnetic (inductive), electrostatic (capacitive), and piezoelectric conversion.
Electrostatic conversion is one of the three conversion mechanisms in which a variable capacitor in an electrostatic energy harvester extracts energy from the work done by vibrations against the electric field formed between the two plates of the charged capacitor. One remarkable benefit of electrostatic harvesters is that they can be microfabricated with MEMS technologies. Therefore, micro- or meso-scale and high precision mechanical structures (variable capacitors) can be manufactured. In addition, micromachined harvesters can be easily integrated with microelectronics, which makes it possible to realize completely autonomous self-powered microsystems.
As the power harvested from vibrations is relatively low, which is typically in the μW to mW range, outputting electric energy as much as possible is the primary goal of energy harvester development. Current device improvements focus on optimizing mechanical structures and power processors (circuits) of vibration energy harvesters for fighting power loss during energy conversion. This invention, however, can realize a high-power output electrostatic energy harvester through a unique multi-layer structure design. This invention presents a new route to increase converted power by increasing the actual achievable power density (AAPD) of an energy harvester.
In vibration energy harvesting, power density is a useful indicator for characterizing and comparing energy harvesters, which is defined in Equation 1. However, it should be used cautiously to avoid being misled. For example, for building a self-powered microsystem containing a certain type of electrostatic micro harvester, a certain space or volume is assigned to be occupied by the harvester. In theory, the maximum converted power would be the product of the assigned volume and the power density of the harvester. This maximum power can be reached only when the microfabricated harvester can fully occupy the assigned volume. However, this is usually not the case because of the limitations of current silicon microfabrication technologies. To more precisely describing this situation, the inventor of this invention has created a term called actual achievable power density (AAPD) defined in Equation 2. Substitution of Equation 1 into Equation 2 yields Equation 3. As Vh is usually less than, in some cases, much less than, Va, pa is therefore less than p. The ratio of Vh/Va reflects the usage ratio of a given volume by a harvester.
                    p        =                  P                      V            h                                              (        1        )                                          p          a                =                  P                      V            a                                              (        2        )                                          p          a                =                              P                          V              a                                =                                                    p                ×                                  V                  h                                                            V                a                                      =                          p              ×                                                V                  h                                                  V                  a                                                                                        (        3        )            where p is harvester power density; pa is actual achievable power density; P is converted power; Vh is harvester volume; and Va is assigned volume.
Equation 3 implies another approach to increase power output in addition to improving harvester mechanical structures and power electronics that are currently being worked on. This additional approach is to increase pa or AAPD by making use of an assigned volume as much as possible, i.e., increasing Vh. Efficiently using an allowed volume is extremely vital to the application of micro energy harvesters in microsystems where space is precious and desired to be made the best use of to its maximum. In some cases, not only is a space for a micro-harvester is restricted, but its footprint area is also restricted as well. A flexible microfabrication technology is therefore strongly required to fabricate complex, arbitrary micro energy harvesters that can occupy a given volume as much as possible.
Currently proposed various dynamic models for vibration energy harvesters show that for a given vibration source the converted power is proportional to the moving mass in a harvester and its travel along with the acceleration. For an electrostatic harvester, vibration energy is converted by a variable capacitor in the harvester when it oscillates between a maximum capacitance Cmax and a minimum capacitance Cmin. The converted energy per cycle is proportional to the capacitance change (ΔC=Cmax−Cmin). The capacitance change is related to the geometry of the variable capacitor. A larger capacitance change is highly desired as it is capable of converting more energy or it can reduce the input voltage and/or the maximum achievable voltage during energy conversion suitable for low voltage circuits.
Reported prototype electrostatic micro energy harvesters fall into three categories: in-plane constant gap comb capacitor type, in-plane gap closing comb capacitor type, and out-of-plane gap closing plate capacitor type.
FIG. 1 schematically illustrates various views of an example of an one-layer in-plane comb capacitor type electrostatic harvester structure. FIG. 1A shows a perspective view of a comb capacitor type electrostatic harvester structure 100 that is anchored to a substrate 102 through four anchors 104, 106, 108, and 110. FIG. 1B shows only the layout of the four anchors on the substrate without the harvester structure. FIGS. 1C and 1D show a perspective view and a top side view of the harvester structure, respectively. The harvester structure comprises two fixed comb finger electrodes 114 and 116, a movable comb finger electrode 112, and two spring structures 118 and 120 that support the movable electrode 112. The two fixed electrodes 114 and 116 are secured to the two anchors 104 and 106, respectively. The two spring structures 118 and 120 are also secured to the two anchors 108 and 110 at two anchor points 122 and 124, respectively so that the movable electrode is suspended above the substrate by the two spring structures. The movable electrode has two sets of comb fingers which form two variable comb capacitors with the two fixed electrodes. The fingers of each comb capacitor are arranged to be interdigitated. The movable electrode also serves as a moving mass. In this harvester structure, the two spring structures are designed so that the movable electrode can only move along the Y-axis (see FIG. 1D) with vibration. Therefore, this harvester structure is an in-plane constant gap comb capacitor type. The gap means a separation between two neighbouring fingers. Those skilled in the art will understand that the spring structures can also be designed so that the movable electrode can only move along the X-axis. Then the harvester structure becomes an in-plane gap closing comb capacitor type. FIG. 1 only shows the mechanical part of the harvester. A workable energy harvester also comprises at least a proper circuitry (not shown in FIG. 1) that connects to the fixed electrodes and the movable electrode so that ambient vibration energy can be harvested and converted to electric energy. Note that the core of the mechanical part of an electrostatic harvester is its variable capacitor structures. The electrostatic energy harvester example shown in FIG. 1 comprises two fixed electrodes, one movable electrode, and two spring structures that forms two variable capacitors. Those skilled in the art will understand that a minimum requirement for forming a variable capacitor only needs one fixed electrode, one movable electrode, and one spring structure. Therefore, in this invention, the mechanical part of an in-plane electrostatic energy harvester comprising at least one fixed electrode, one movable electrode, and at least one spring structure can be generally called a variable capacitor layer.
For the in-plane comb capacitor type electrostatic energy harvester, increasing converted power requires increasing the thickness and the aspect-ratio of the comb capacitor, meaning increasing mass and capacitance. To fulfil the above requirements, DRIE (deep reactive ion etching) has become the most suitable microfabrication process for making silicon-based single-layer comb capacitor type electrostatic energy harvesters. Even though DRIE can produce high-aspect-ratio silicon structures, its reasonable aspect-ratio is only up to 50. For example, for an in-plane constant gap capacitor having a gap separation of 4 μm, the maximum silicon layer thickness would be less than 200 μm. Note that the capacitance change is inversely proportional to the gap separation. Therefore, we have a thickness constrain. To further increase converted power, we have to increase the footprint area of the harvester. However, in a microsystem, an allowed footprint area provided for a harvester is usually also constrained. In addition, DRIE is not a truly three-dimensional (3-D) fabrication process. Rather, it is a 2.5-D process, meaning that either the cross-section of a silicon structure along z-axis is exactly the same or its geometry has very limited change along z-axis. Lacking of geometry change along z-axis restricts DRIE to form complex silicon structures which are essential for a harvester to fill up a given volume. Summarily, unless an assigned volume in a microsystem is exactly the same as a harvester that can be made with DRIE (a very rare situation), the fabricated harvester cannot usually fully use the given volume. FIG. 2 schematically illustrates such a situation where an one-layer comb capacitor type electrostatic energy harvester structure 126 (its spring structures not shown) only occupies a portion of a given volume 128, meaning a low AAPD and therefore a low converted power in terms of this given volume.
A logical solution for an energy harvester to occupy more volume is to stack more one-layer variable capacitor structures to form a multi-layer variable capacitor structure such as a three-layer example shown in FIG. 3 where three identical one-layer variable capacitor structures 126, 132, and 134 (its spring structures not shown) are bonded together in the same given volume 128.
Bonding of pre-micromachined silicon structures has become a standard practice in MEMS fabrication. One well-known example is the MIT's micro gas turbine engine fabricated by bonding five layers of pre-micromachined silicon wafers. However, the difficulty of forming a multi-layer electrostatic harvester is that its each variable capacitor layer contains discrete silicon features shown in FIG. 4 where two fixed electrodes 138 and 140 do not mechanically connect with a moving electrode 142 on the same layer. Obviously this kind of layer cannot be made separately as discrete features will fall down. This is a fundamental drawback in standard bulk micromachining plus silicon bonding. To overcome this technical barrier, special fabrication approaches have to be developed. In the case of MIT's micro-engine, for making the layers containing discrete features, one solution is to use temporary silicon connections which support discrete silicon features. After all layers are bonded together, the temporary silicon connections are either cut with a laser beam or mechanically fractured to release the discrete features. However, this post separation treatment only works for special designs. In addition, it is not a reliable and desirable approach as the removal of the silicon mechanical connections is the last step of the process. If it fails, all previous work will be wasted. To avoid making the layers containing discrete features, another solution is to do multiple etching on both sides of silicon layers. However, DRIE can be performed only once on each side of a layer, which restricts to form more complex structures. Besides, this solution involves many process steps and may face difficult operations, low throughput and low yield.
As the converted power is proportional to the moving mass of an energy harvester, using silicon as the material of harvesters has an intrinsic drawback due to its low density (2.33 g/cm3). Therefore, attaching a high-density material on the movable electrode of an electrostatic harvester is highly desirable. Miao et al. proposed to electroplate a layer of gold (density: 19.3 g/cm3) on the silicon movable electrode of their out-of-plane gap closing type electrostatic energy harvesters. It is not difficult to electroplate a metal on a silicon plate capacitor. But it would be quite challenging to electroplate a layer of metal just on the silicon movable electrode of a microfabricated comb capacitor type harvester. Roundy proposed to attach a piece of a tungsten alloy (density: ˜17 g/cm3) on his comb capacitor type silicon harvester. However, making a precise micro-scale tungsten alloy mass and then precisely attaching it on a micromachined silicon harvester seem not to be a feasible mass production approach.